1. Field of the Invention
The present invention relates to a dispersion compensating fiber (DCF) for an S-band discrete Raman amplifier (RA).
2. Discussion of Related Art
Nowadays, as there have been intensive studies on the extension of the transmission band as part of a method for increasing transmission capacity, much attention has been paid to transmission techniques in the S-band, which is a low-loss band, as well as conventional C/L-band. Here, a loss characteristic of a silica fiber according to the wavelength is illustrated in FIG. 1. Generally, the C-band ranges from 1525 to 1565 nm, the L-band ranges from 1565 to 1605 nm, and the S-band ranges from 1485 to 1525 nm. In the C- or L-band, optical signals can be amplified using an erbium doped fiber amplifier (EDFA). However, a conventional EDFA cannot amplify optical signals in the S-band because it has no gain in the S-band.
There are a rare-earth-element doped fiber amplifier such as a thulium doped fiber amplifier (TDFA), a Raman amplifier (RA) using Raman scattering in optical fibers, and a semiconductor amplifier, as amplifiers having gain in the S-band.
The TDFA employs thulium doped fibers (TDFs), which are made by doping thulium into nonsilica-based glass (i.e., fluoride glass), as amplification mediums. Thus, when the TDFA is connected to common silica fibers, great loss and problems are generated.
With the development in the manufacturing technology of semiconductor amplifiers, the semiconductor amplifiers are on the rise. However, a lot of technical limitations, for example, small gain and high polarization mode dispersion (PMD), still remain unsolved.
The RA has the advantage of having gain in various bands according to the pump wavelength, so it is appropriate for the S-band in which optical signals cannot be amplified using an EDFA. Above all, unlike a distributed RA that enables the Raman pumping of transmission lines, a discrete RA is inserted into a transmission system as an additional element to achieve gain. An S-band discrete RA includes pump laser diodes (LDs), optical fibers as amplification mediums, and other optical devices, such as a wavelength division multiplexer (WDM) that couples input optical signals and pump signals. So far, the gain of the S-band discrete RA has been controlled by adjusting the wavelength and output power of the pump LDs and the length of dispersion compensating fibers (DCFs). Accordingly, the S-band discrete RA cannot achieve high gain due to a limited output power of the pump LDs and the high loss of C-band DCFs and also has no adequate dispersion characteristics.
In this regard, a method of increasing gain by using nonlinear fibers having a high Raman gain coefficient along with conventional DCFs was introduced by David J. DiGiovanni [“Raman Amplified Dispersion Compensating Modules”, U.S. Pat. No. 6,504,973 B1, Jan. 7, 2003]. However, it is troublesome that the nonlinear fibers be designed and manufactured considering the dispersion of conventional DCFs, and it is difficult to expect a substantial increase in gain because the nonlinear fibers cause greater loss than the conventional DCFs.